A student performs an experiment to observe Brownian motion using a smoke cell apparatus. Fig. 66.1 shows the complete setup. A glass cell containing smoke ...

Part (a) [2 marks]

Brownian motion is the random, erratic movement of small visible particles [1] caused by collisions with invisible molecules that are in constant random motion [1].

The key idea is that the visible particles (smoke, pollen grains, etc.) are not moving on their own. Their motion is evidence of the invisible molecular bombardment happening around them.

Part (b) [2 marks]

Tiny bright specks (points of light) are seen through the microscope [1]. They move in a random, jerky, unpredictable way, constantly changing direction [1].

The bright specks are smoke particles reflecting light from the lamp. They appear bright against a dark background because only the light scattered by the particles reaches the microscope eyepiece.

Part (c) [2 marks]

• J is the lamp (light source) [1]
• K is the microscope eyepiece [1]

The lamp illuminates the smoke particles from the side so they appear as bright specks against a dark background (dark-field illumination). The microscope eyepiece allows the observer to view the tiny particles at sufficient magnification.

Part (d) [3 marks]

The visible smoke particles are being struck by invisible air molecules [1]. The random changes in direction of the smoke particles show that the air molecules move in random directions [1]. The continuous, unceasing motion demonstrates that air molecules are in constant motion, not stationary [1].

If the air molecules were still, the smoke particles would slowly settle downward under gravity. Instead, they are constantly jostled in random directions, proving that the surrounding molecules are always moving and colliding with them from all sides.

Part (e) [2 marks]

Using \(KE = \frac{1}{2}mv^2\):

\(KE = \frac{1}{2} \times 4.8 \times 10^{-26} \times (500)^2\) [1]

\(KE = \frac{1}{2} \times 4.8 \times 10^{-26} \times 250\,000\)

\(KE = 6.0 \times 10^{-21}\) J [1]

This is an extremely small amount of energy for a single molecule, but there are vast numbers of molecules (on the order of 10²³) in even a small volume of air, so their collective kinetic energy produces measurable effects like pressure and temperature.